The present invention relates to a method of detecting carbides in alloy steels that are related to creep damage which occurs in a hot and stress-loaded state.
More than 70% of the steam turbines currently in operation have been in service for periods longer than 10 years. Since the parts of those turbines are used at elevated temperatures, the development of a reliable diagnostic technique for predicting the remaining life of turbine parts by measuring the degree of material degradation in a nondestructive manner is important.
The CrMoV steel used as the structural material of steam turbines is subject to various types of damage, including "creep damage" that occurs if stress is loaded under the high-temperature condition, "fatigue damage" that results from repeated force, and "temper damage" that is characteristic of the CrMoV steel.
Of these damages, the "creep damage" requires the most serious consideration. The CrMoV steel is provided with high strength by dispersing fine carbides and dislocation structures in the metal matrix. However, as the steel is used at high temperatures for a long-term period, material degradation will occur on account of such factors as the precipitation and coarsening of carbides and the recovery of dislocation structures and it is accelerated by the action of a stress load. Furthermore, voids will develop at grain boundaries and connect one another until a crack occurs. Two methods are used today as nondestructive techniques for measuring the creep damage: they are i) evaluation by the degree of softening in the material hardness (which is hereunder referred to as a "hardness method") and ii) evaluation by the amount of creep voids that develop at grain boundaries (which is hereunder referred to as a "creep void method").
The conventional "hardness" method and "creep void" method have had the following problems. With the hardness method, the initial hardness of the specimen in the virgin state is used as the reference for evaluating the degree of degradation in the specimen in terms of either the amount of hardness drop that occurred as a result of long-term service or the ratio of hardness drop (the measured value divided by the initial value). It is therefore necessary for the success of the hardness drop method to predetermine the initial value of the hardness of the specimen and, if the initial value is unknown, precise estimation of the initial value is critically important. In most cases, the value of hardness of an undamaged area (e.g., the coupling portion of a steam turbine's rotor which is used at low temperatures) is substituted as the initial value but in the case of a structural member having a distribution in the values of hardness (which hence are not uniform), the initial value of the hardness of the area to be evaluated cannot be estimated with high precision.
The creep void method has the inherent problem that the conditions for the formation of voids and the measurement values are subject to variations and, hence, the method is suitable for evaluating the degree of degradation in specimens that are in the second half of the progress of creep damage (&gt;40 to 50%), with 100% creep damage corresponding to the occurrence of creep rupture. However, in view of the fact that most of the machines in current service that need be evaluated for remaining life are found to have experienced approximately less than 40% creep damage on the basis of theoretical estimation, one may well say that the creep void method does not guarantee satisfactory precision in evaluation. A further problem with the creep void method is its low operational efficiency since the microscopic metallurgical structure of the specimen's surface need be transferred to a replica film, which is then subjected to a measurement in laboratory using a suitable instrument such as a scanning electron microscope.